When converting a feedstock containing a hydrocarbon to a product in an industrial reactor, it is desirable to maximize the production of a desired product or products, and to control, typically to minimize, the production of by-products. One type of reactor useful for conducting hydrocarbon conversion reactions is a fluidized bed reactor, wherein solid catalyst particles are suspended in a fluidized state during contact with the feedstock and other vapor materials. These types of reactors generally have a regular cylindrical geometry, that is, comprise a cylinder of some length having a constant diameter along that length. In general, the reactor cross-sectional area available through which catalyst particles and vapors flow is directly proportional to a characteristic width, and specifically for a cylinder directly proportional to the diameter.
U.S. Pat. No. 6,166,282 discloses one type of fluidized bed reactor which is used in converting oxygenates to olefins. The reactor includes a dense phase zone, a transition zone, and a riser section. It is disclosed that by using the dense phase zone and the transition zone in the same reactor, combined with an initial catalyst separation or disengagement step, significant advantages over conventional bubbling or fluidized bed reactors can be obtained. The dense phase zone operates at a superficial velocity of less than 1 meter per second, and the transition zone operates at a superficial velocity of from 1 to 4 meters per second. As a mixture of unreacted feedstock, fluidized catalyst particles, and reaction products enters the transition zone, the reaction continues to essentially complete conversion.
European Patent Application EP 1 046 695 discloses another type of fluidized bed reactor which incorporates the use of multiple zones in a fluidized bed reactor. This patent publication is specifically directed to the utilization of different feedstock and other vapors in each of these zones to control reaction conditions such as temperature in each of these zones, and to increase secondary reactions and byproduct production.
PCT Publication WO01/04079 discloses a fluidized bed reactor having a fluidization section and a riser section. This patent publication is specifically a reaction system involving oxidation of propylene with a vanadium and bismuth catalyst to produce acrylic acid.
An article in the Chinese periodical Petroleum Processing and Petrochemicals, Vol. 31, No. 2, February 2000, by Zheng Maojun, et. al., entitled “Determination Of Particle Velocity Distribution In Two Kinds Of Riser Reactor”, discusses an auxiliary bed of a larger diameter in conjunction with a riser of smaller diameter. The article describes the use of the auxiliary bed as a means to provide a better plug flow pattern in the riser.
Additional systems and methods are needed to improve the effectiveness of fluidized bed hydrocarbon conversion reactors. In particular, such systems are needed to improve product selectivity while maintaining a high conversion of feed. Such systems are particularly desirable in maintaining high selectivity to ethylene and propylene products in the emerging field of oxygenate conversion using oxygenate to olefin catalysts.